1. Technical Field
Embodiments of the present invention relate to a sheet conveyor, an image forming apparatus incorporating the sheet conveyor therein, and a method of preventing sheet skew in the sheet conveyor.
2. Related Art
As an example of sheet conveyance in known image forming apparatuses, sheets that function as transfer recording media loaded on a sheet conveying device are fed one by one as a sheet feed roller rotates to convey the fed sheet to an image transfer position in the image forming apparatus. The sheet fed by the sheet feed roller receives a toner image thereon at the image transfer position while a position of the toner image formed on a photoconductor drum or a photoconductor belt matches an image transfer position of the sheet. After the toner image is transferred onto the sheet and is fixed to the sheet by application of heat and pressure, the sheet is discharged as a print or copy.
To transfer the toner image onto the sheet with precise positioning, these image forming apparatuses generally include a timing mechanism to match a timing in which the sheet is fed with a timing in which the toner image arrives at the image transfer position in the image forming apparatus. Examples of the timing mechanism are disclosed in Japanese Unexamined Utility Model (Registration) Application Publication No. JP S64-000555-A and Japanese Patent Application Publication No. IP H05-338865-A.
JP S64-000555-A discloses a configuration of an image forming apparatus including a gate member. The gate member is disposed upstream from a timing roller pair that conveys a sheet according to a transfer timing. The gate member can freely advance/retreat with respect to a sheet conveying path.
JP H05-338865-A discloses a configuration of an image forming apparatus that does not include the gate member but includes a mechanism to abut a leading edge of a sheet forcedly against a nip contact area that is formed between rollers of a timing roller pair.
By temporarily continuing the sheet conveyance while the leading edge of the sheet is once pressed against the gate member or the nip contact area, a part of the sheet slacks to form a sag or a curve in the sheet. And, at the same time, skew of the sheet is corrected by pressing the leading edge of the sheet against the gate member or the nip contact area. The skew of the sheet is caused due to difference in accuracy in parts or components such as sheet conveying rollers and/or due to incorrect setting of the sheet with respect to the sheet conveying device. For these reasons, a skew correction mechanism is demanded.
Japanese Patent Application Publication No. JP 2003-118890-A discloses a configuration of a sheet conveying device that eliminates deviation of amounts of slacks in a sheet or a slack amount deviation of a sheet. The sheet conveying device disclosed in JP 2003-118890-A includes two drive rollers and two respective driven rollers. The drive rollers and the driven rollers are individually slidable in an axis direction of respective shafts of the drive and driven rollers.
Each of the rollers rotates idly when receiving a force applied in the sheet conveying direction PD and remains unrotated when receiving a force applied in an opposite direction of the sheet conveying direction PD. According to this configuration, the sheet can easily move in a left or right direction immediately when the slack of the sheet is formed. This movement of sheet can eliminate the slack amount deviation at the left and right sides of the sheet.
However, the sheet conveying device disclosed in JP 2003-118890-A may also include a return spring to return a roller or rollers slid in the axial direction to an initial center position (a home position). A force applied by the return spring constantly acts in a thrust direction of the rollers during image transfer. Therefore, the sheet during image transfer rotates by the force in the thrust direction, and this can result in production of the trapezoidal defect image.
Further, as an alternate member to the return spring, a more complex mechanism may be employed so as not to apply the force in the thrust direction of the rollers during image transfer, which results in an increase in cost. In addition, the rollers to the initial center position (the home position) may need to return while the sheet is not passing through the roller during intervals of sheets, and therefore high printing performance cannot be achieved.